1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method for controlling transmission power to stably transmit data or control information to a Base Station (BS).
2. Background Art
Future-generation multimedia wireless communication systems, which are a recent active research area, require processing of various types of information including video data and wireless data at high data rates, beyond the traditional voice-oriented service.
Owing to its advantage of high data rate, Orthogonal Frequency Division Multiplexing (OFDM) has recently attracted much attention. OFDM is a special case of Multi-Carrier Modulation (MCM) in which a frequency band is divided into multiple orthogonal subcarriers and data is transmitted on the subcarriers. The low complexity of OFDM reduces inter-symbol interference. In OFDM, an input serial sequence of data symbols is converted to N parallel data symbols and transmitted on N separate subcarriers.
Orthogonality is maintained among the subcarriers in the frequency domain. Each orthogonal channel experiences mutually independent frequency-selective fading and the resulting increase in the gap between transmitted symbols leads to minimum inter-symbol interference. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme in which multiple accesses are allowed through independent allocation of part of available subcarriers to each user in an OFDM system. In OFDMA, it is typical to independently allocate frequency resources, namely subcarriers to a plurality of users in such a manner that the frequency resources are not overlapped with one another. As a result, allocation of frequency resources is mutually exclusive among users in OFDMA.
For an efficient system configuration, a wireless communication system is designed to be cellular. A cell is defined as a smaller area divided from a larger area to efficiently use frequency. In general, a BS is deployed at a cell center to communicate with User Equipments (UEs). A cell is the coverage area of a BS.
The wireless communication system adopts a power control scheme as one of ways to reduce path loss that depends on the distance between a BS and a UE and inter-cell interference from neighbor cells. According to the power control scheme, the transmission power of data is controlled to be low enough to maintain Quality of Service (QoS) at a certain level. Especially UEs at a cell edge are greatly affected by path loss and inter-cell interference in a multi-cell environment. A UE should transmit data at an appropriate transmission power level that does not decrease QoS according to path loss without causing interference to neighbor cells.
Accordingly, the UE determines its transmission power based on path loss, inter-cell interference, and a target Signal-to-Interference plus Noise Ratio (SINR) for data transmission. For the determination of the transmission power, the UE also considers UE-controlled UE-specific power correction and BS-controlled UE-specific power correction. The UE calculates the target SINR using target SINR parameters received from the BS. How to determine the target SINR depends on an operation mode. In Mode 1, the UE determines the target SINR based on a number of control factors that are received from the BS for use in controlling a total system throughput, a trade-off between operations at a cell edge, and an Interference plus noise over Thermal noise (IoT), whereas in Mode 2 the UE determines the target SINR based on a Carrier-to-Noise Ratio (C/N) shared between the BS and the UE for each transmission channel on which the UE transmits data and/or control information to the BS. The problem of the UE's complexity has been brought up from the method for determining a target SINR differently for different modes. Especially in Mode 2, the C/N used in determining the target SINR is determined based on the offset of each piece of information, Ioffset. Considering this information offset reflects a resource size that varies with a Modulation and Coding Scheme (MCS) level or a channel status different even for the same information, the determination of the C/N of each transmission channel is very complicated and difficult.
The UE adjusts its transmission power using a UE-determined UE power offset as well as a UE power offset received from the BS. Because the channel environment of the UE changes very fast, power control based on UE-transmitted feedback information may adversely affect the stability of the channel environment of the wireless communication system.
Accordingly, there exists a need for a method for determining transmission power so as to maintain the stability of the channel environment of a wireless communication system, while simplifying the process of determining transmission power at a UE.